Synthesis, structural characterization, solvatochromism, and electrochemistry of tetra-osmium carbonyl clusters containing azo-ligands

The reaction of [Os4(μ-H)4(CO)12] with 4-(2-pyridylazo)-N,N-dimethylaniline (PNNDA) in dichloromethane afforded the new clusters [Os4(μ-H)4(CO)11NC5H4(N=N)C6H4NMe2}] (1) and [Os4(μ-H)4(CO)10{η2-NC5H4(N=N)C6H4NMe2}] (2) in 34% and 15% yields, respectively. Upon heating in toluene under reflux, compound 1 converted into 2 in 80% yield. The MLCT transition of compound 2 shows strong solvent dependency, displaying unusually large positive solvatochromism in different organic solvents. Treatment of [Os4(μH)4(CO)12] with 2-(5-bromo-2-pyridylazo)-5-(diethylamino)phenol (Br-PADAP) in dichloromethane gives [Os4(μ H)3(CO)10{μ-η3-Et2NC6H3(O)(N=N)C5NH3Br}] (3). The O - H bond activation is observed and the azo-ligand behaves as a five-electron donor. The clusters [Os4(μ-H)2(CO)10{μ-η3NC5H4(N=N)C5H4N}] (4) and [Os4(μ-H)4(CO)10{μ-η2NC5H4(N=N)C5H4N}]2(5) were isolated from the reaction of [Os4(μ-H)4(CO)10(NCMe)2] with 2,2′-azopyridine and 3,3′-azopyridine, respectively. In compound 4, both pyridine and azo nitrogen atoms were found to coordinate to the cluster core. Dehydrogenation was also observed in this compound. For compound 5, two tetra-osmium metal cores were connected by two azo ligands through their pyridyl nitrogen atoms to form a novel cyclic structure. The redox properties of the compounds described herein were investigated by means of cyclic voltammetry and controlled potential coulometry. Both compound 2 and 5 exhibit a reversible cathodic wave, which indicates that they undergo addition of one electron without significant structural changes. Furthermore, compound 5 displays an electronic interaction between two redox centers, the extra electrons are believed to delocalize throughout the cyclic structure.